Fiber optic communications networks may increase the amount of information carried on an optical fiber by multiplexing different optical signals on different wavelengths using wavelength division multiplexing (WDM). In a WDM passive optical network (PON), for example, a trunk fiber carries optical signals at multiple channel wavelengths to and from an optical branching point and the branching point provides a simple routing function by directing signals of different wavelengths to and from individual subscribers. In this case, each subscriber may be assigned one or more of the channel wavelengths on which to send and/or receive data.
To transmit and receive optical signals over multiple channel wavelengths, an optical line terminal (OLT) in a WDM-PON may include a multi-channel transmitter optical subassembly (TOSA) and a multi-channel receiver optical subassembly (ROSA). One example of a TOSA includes a multiplexed laser array that combines multiple optical signals at multiple channel wavelengths. In laser transmitters, the lasers may be monitored to determine that the lasers are operating with the desired power and at the desired wavelength. Monitoring lasers in laser transmitters has involved a photodetector coupled to each of the lasers. In multiplexed laser arrays, however, it may not be feasible to couple a photodetector to each of the lasers, for example, within the limited space of a multi-channel TOSA. The photodectors directly coupled to the multiplexed lasers also may not accurately monitor the wavelength of the light after passing through the optical multiplexer.